1. Field of the Invention
The present invention relates to a clamping mechanism for a tool holder.
2. Description of the Related Art
Year after year, in order to improve machining efficiency, revolution speed of the main spindle of a machine tool has become faster, and it has become a common practice to exceed 20,000 rpm in maximum speed. In such a main spindle at high speed revolution, how low vibration during the main spindle revolution can be kept in terms of noise, reliability and machining precision becomes important.
As the main spindle speed becomes faster, the structure capable of keeping rotational vibration of the main spindle low has been requested even for a tool clamping mechanism to be provided within the main spindle.
FIG. 5 is a view showing a structural example of a main spindle mechanism using a conventionally-known clamping mechanism. In FIG. 5, a clamping mechanism 1 (1a to 1e) is provided within a main spindle 2. The main spindle 2 is rotatably mounted on a housing 5 via a bearing 6.
The clamping mechanism 1 is composed of: a drawbar 1a; a steel ball 1b; a nut 1c coupled to the drawbar 1a; a seat plate 1d and a disk spring 1e. 
The tool holder is composed of the main body 4a of a tool holder for holding a tool such as a cutter 4b, a pull stud 4c, and a tool shank 4d. The tool shank 4d is inserted into an end portion open end of the main spindle 2 by an automatic tool exchanger and the steel ball 1b is extended by the pull stud 4c at the upper end to thereby mount the tool.
The clamping mechanism 1 is capable of assuming two states: a tool holding state for holding the main body 4a of tool holder; and an unclamped state in which the tool holder has been unclamped. The clamping mechanism 1 draws, in the tool holding state, the main body 4a of tool holder into the main spindle 2 side for holding by transmitting a repulsion force of the disk spring 1e compressed to the pull stud 4c via the nut 1c, the drawbar 1a and the steel ball 1b. 
Also, when unclamping the main body 4a of tool holder, a roller 7 is operated and a ring 3b coupled to the nut 1c via a rod 3a is pressed down against the repulsion force of the disk spring 1e. Thereby the nut 1c, the drawbar 1a and the steel ball 1b advance (descend) together, and the steel ball 1b moves to an air gap 2a within the main spindle 2 to unclamp the pull stud 4c. 
Such conventional structure as described above presents no problem particularly when the revolution speed of the main spindle is comparatively low, but when the main spindle speed reaches such high speed revolution as to exceed 20,000 rpm, a problem that the rotational vibrations (vibration acceleration, vibration amplitude) could not be kept low has come to the surface.
As a main cause for vibrations during revolution of the main spindle, assembly precision of the main spindle including the bearing, mass imbalance of the main spindle rotating part, resonance of the structure and the like are conceivable.
The clamping mechanism preferably keeps the rotational vibrations low due to the mass imbalance of the main spindle rotating part not to be increased or no resonance to be caused at specific main spindle speed, and long operational life is desirable.
In order to realize their ideal states, however, the conventional clamping mechanism has the following problems.
The disk spring for drawing up the tool holder to hold it is capable of being installed within a small space, and has an advantage of being able to exhibit a high spring force, but on the other hand, has the following problems.
(1) Since a guide clearance between the inner diameter of the disk spring and the outer diameter of the drawbar cannot be made sufficiently small, the position of its center of gravity changes due to repeated clamp/unclamp operation of the tool holder and repeated revolution/stop of the main spindle.
For the reason, even if the vibrations have been kept low by accurately modifying the mass imbalance of the entire main spindle in initialization, the maintenance and the like, discrepancy in an adjusted state due to balance modification against mass imbalance will be caused by repeating the clamp/unclamp operation or the revolution/stop of the main spindle thereafter, so that the amplitude of the vibrations will become large again.
(2) Frictional resistance to be caused among a plurality of disk springs provided causes non-uniformity to expansion and contraction strokes of the individual springs. When the expansion and contraction strokes become non-uniform, there will locally exist some disk springs having large stress amplitude among a plurality of dish springs as one set. The repetition life of the compression/release operation will become short.
(3) Since the disk spring is flat in shape, a length to guide in the axial direction in an inner diameter portion of the disk spring is short. For the reason, sliding of the disk spring in the axial direction is not smoothly performed, but an inner diameter portion of the disk spring and an outer diameter portion of the drawbar wear each other, possibly suffering damage.
As a method for solving problems resulting from the disk spring of the above-described problems (1) to (3), there has been proposed a clamping mechanism using a coil spring (See, for example, Japanese Patent Application Laid-Open No. 2000-296404).
In the clamping mechanism using the coil spring, however, the guide clearance between the inner diameter of the coil spring molded and the drawbar guide cannot be made small, every time the clamp/unclamp operation of the tool holder is repeated, and every time the revolution/stop of the main spindle is repeated, the position of the center of gravity changes. For the reason, even if the vibrations have been kept low by accurately modifying the mass imbalance of the entire main spindle, there is a problem that if the clamp/unclamp operation or the revolution/stop of the main spindle is repeated thereafter, the adjusted state due to balance modification will be out of order and the amplitude of the vibrations will become large again. Also,
(4) In addition to the above-described problems, there is a problem that the position of the tool clamping mechanism always becomes unstable during revolution at high speed.
Since usually the clamping mechanism requires an operation for expanding and contracting the mechanism for the clamp/unclamp operation of a tool, the clamping mechanism is constructed to be sliding-guided with a fixed clearance with respect to the inner wall of the main spindle. This clearance causes the drawbar and the laminated disk springs to vibrate in the direction of diameter, so that the position of the tool clamping mechanism always becomes unstable, and vibrations occur. These vibrations become remarkable as the main spindle speed increases.
As a method for keeping low the vibrations of the drawbar of the above-described problem (4) in the direction of diameter, a method for providing the drawbar and the inner wall of the main spindle with thrust bearings has been disclosed in Japanese Utility Model Application Laid-Open No. 5-63701. According to this method, it is considered possible to guide a draw bar on the inner wall of the main spindle with high accuracy without bending the drawbar during revolutions of the main spindle at high speed.
In order to make a smooth operation of the thrust bearing and the guidance of the drawbar compatible, however, there is a necessity for a minimal (for example, several μm) press-fit allowance between the outer diameter of the drawbar, the outer diameter of the ball and the inner diameter of the main spindle respectively. In order to provide such a press-fit allowance, it is necessary to manage the respective part dimensions with exceedingly high precision, leading to a problem that the cost will be increased.